Rebjorn - I think that's a fantastic idea and one that would truly benefit the community and the players. Thinking about this makes me wonder why Blizzard hasn't done something along these lines (though, I suppose I wouldn't consider them particularly supportive to mods, unlike other developers like Bethesda). If Blizzard is truly as profit-driven as it seems to us, that leads to the question: if there were a profitable way to do this, why haven't they done it?
Certainly, there is a degree of risk involved in greater support for modding / releasing source code, but I imagine the potential for profit could easily outweigh these risks, especially considering D2 is an older property. Perhaps, then, the potential for profit isn't as significant as we believe. I find it doubtful they haven't at least considered possibilities like this - so perhaps they have done analysis and found that the profits just aren't there for some reason (or the risks are larger than we have considered).
Maybe, for example, they are concerned about drawing attention away from their newer properties (i.e. D3, D4, Diablo Immortal, etc) by shifting player focus back onto D2 - cannibalizing some of their own audience. This could make sense, considering they are surely aware of the legacy of D2 compared to the somewhat lukewarm perception of D3, D4, and Immortal.
They also may simply prefer to maintain greater control over their properties - something which supporting modding on an older title might undermine. It's harder to justify DRM, paid DLC's, and micro transactions on a new title if you're actively supporting modding on an older (arguably, more popular) title. This higher level of control and propriety over games is, unfortunately, gaining in popularity. It may be less beneficial for players, but it is a safer bet for the developer and shareholders.
Ultimately, I think that your idea is great - arguably, better than Blizzard attempting a remaster / remake (as I agree that the fanbase and modders are more likely to put in the effort to crafting a more soulful and honest experience) - but it would likely require a major shift in Blizzard's direction... something which isn't likely to happen if they're content and profiting off of their current model.
In short, this is a great idea but, if Blizzard is as greedy as it seems, I doubt it will happen (sadly). Not enough control over the product, maybe too much doubt about losing audience from their newer games.